1. Field of the Invention
The present invention relates to the formation of dielectric layers during fabrication of integrated circuits on semiconductor wafers. More particularly, the present invention relates to a method for providing a dielectric film having a low dielectric constant that is particularly useful as a premetal or intermetal dielectric layer.
2. Description of the Related Art
One of the primary steps in the fabrication of modern semiconductor devices is the formation of a thin film on a semiconductor substrate by chemical reaction of gases. Such a deposition process is referred to as chemical vapor deposition or xe2x80x9cCVD.xe2x80x9d Conventional thermal CVD processes supply reactive gases to the substrate surface where heat-induced chemical reactions take place to produce a desired film. Plasma enhanced CVD techniques, on the other hand, promote excitation and/or dissociation of the reactant gases by the application of radio frequency (RF) or microwave energy. The high reactivity of the released species reduces the energy required for a chemical reaction to take place, and thus lowers the required temperature for such PECVD processes.
Semiconductor device geometries have dramatically decreased in size since such devices were first introduced several decades ago. Today""s fabrication plants are routinely producing devices having 0.25 xcexcm and even 0.18 xcexcm feature sizes, and tomorrow""s plants soon will be producing devices having even smaller geometries. In order to further reduce the size of devices on integrated circuits, it has become necessary to use conductive materials having low resistivity and insulators having a low dielectric constant. Low dielectric constant films are particularly desirable for premetal dielectric (PMD) layers and intermetal dielectric (IMD) layers to reduce the RC time delay of the interconnect metalization, to prevent cross-talk between the different levels of metalization, and to reduce device power consumption. Undoped silicon oxide films deposited using conventional CVD techniques may have a dielectric constant (k) as low as about 4.0 or 4.2. One approach to obtaining a lower dielectric constant is to incorporate fluorine in the silicon oxide film. Fluorine-doped silicon oxide films (also referred to as fluorine silicate glass orxe2x80x94xe2x80x9cFSGxe2x80x9d films) may have a dielectric constant as low as about 3.4 or 3.6. Despite this improvement, films having even lower dielectric constants are highly desirable for the manufacture of integrated circuits using geometries of0.18 xcexcm and smaller. Numerous films have been developed in attempts to meet these needs including: a spin-on glass called HSQ (hydrogen silsesqui-oxane, HSiO1.5) and various carbon-based dielectric layers, such as parylene and amorphous fluorinated carbon. Other low-k films have been deposited by CVD using an organosilane precursor and oxygen to form a silicon-oxygen-carbon (Sixe2x80x94Oxe2x80x94C) layer.
While the above types of dielectric films are useful for some applications, manufacturers are always seeking new and improved methods of depositing low-k materials for use as IMD and other types of dielectric layers. For example, after deposition at low temperature, a silicon carbon or Sixe2x80x94Oxe2x80x94C film is often quite porous. Consequently, the film tends to absorb moisture. The absorbed moisture generally degrades the properties of the film. In the case of a low-k film, moisture tends to increase the dielectric constant of the film and is detrimental to film adhesion. The porosity is normally reduced during the previously described thermal cure. However, if the cure is performed ex-situ, the film is exposed for a time to moisture from the ambient atmosphere.
The method of the present invention provides a new and improved post-deposition treatment process. The method of the present invention deposits and densities and cures an insulating layer. The post-deposition densification treatment further enhances adhesion by reducing shrinkage of the deposited film. The post-deposition densification takes place in a reducing environment. In one embodiment, the deposited film is treated in a reducing environment of ammonia for approximately 1 to 5 minutes at a temperature of approximately 400xc2x0 C. Curing can be done in either a vacuum or conventional furnace environment. The densification is particularly useful for enhancing the stability of a film that is to be cured ex-situ, i.e. after removing the substrate from vacuum.
The densification is beneficial for films deposited by a low-temperature CVD process. In one embodiment, the insulating layer is deposited from a process gas of ozone and an organosilane precursor having at least one silicon-carbon (Sixe2x80x94C) bond. During the deposition process, the substrate is heated to a temperature less than about 250 C. In some embodiments, the organosilane precursor has a formula of Si(CH3)xH4-x where x is either 3 or 4 making the organosilane precursor either trimethylsilane (TMS) or tetramethylsilane (T4MS). In other embodiments, the substrate over which the carbon-doped oxide layer is deposited is heated to a temperature of between about 100-200xc2x0 C. and the deposition is carried out in a vacuum chamber at a pressure of between 1-760 Torr.
The method is particularly useful in the manufacture of sub-0.2 micron circuits as it can form a PMD or IMD film with a dielectric constant below 3.0. The film has good gap fill capabilities, high film stability and etches uniformly and controllably when subject to a chemical mechanical polishing (CMP) step. The method is particularly useful when, for throughput reasons, the substrate containing the deposited film is removed from vacuum for curing in a furnace.
These and other embodiments of the present invention, as well as its advantages and features, are described in more detail in conjunction with the text below and attached figures.